On the correspondence between surface UV observations and TOMS determinations of surface UV: a potential method for quality evaluating world surface UV observations

Abstract

A comparison of erythemally weighted surface UV irradiance observations with similar NASA TOMS surface UV
determinations is described. Comparisons are made for two observation periods: the Robertson-Berger (R-B) meter period
from 1974 to the late 1980s and the current period from 1996 to the present when more sophisticated UVB-1 instruments
were used. The more primitive R-B meter observations that comprised the fi rst U.S. UV network are seen to drift downward
with respect to those of the TOMS. While the UVB-1 observations did not appear to drift, a substantial bias is noted to exist
between the TOMS and the UVB-1 stations collecting observations; the TOMS estimations tend to be higher. A portion of the
bias may be attributed to errors in calibration, total ozone, and cosine response of the surface instrumentation. Unaccounted
aerosol effects, although not considered to be large in the TOMS estimations, present another source of error. Comparisons
are fi rst done for all sky conditions and then for clear sky conditions. The biases typically agree for all sky conditions
within the uncertainties of the surface instruments' calibrations, liberally defi ned as ± 5%, implying that the TOMS cloud
correction scheme performs reasonably well. Snow cover severely impacts the TOMS observations, giving considerably
higher estimations. The biases for clear sky conditions ranged from 15% to 19% with no obvious drifts between the satellite
and surface observations. The variation in the biases among stations is within the calibration uncertainties of the instruments,
but the absolute bias is unexpectedly large. The standard deviations of the clear sky comparisons among all stations are steady
at 4.8% ± 0.7%. A plot of the TOMS/UVB-1 ratio versus TOMS cloud refl ectivity observations is noisy, but qualitatively
suggestive of a possible slight increase (~ 5% or greater) over the range of clear to overcast skies. The results from these
comparisons is believed to be relevant to a WMO goal of uniformly assuring the quality of UV observations made by networks
in many countries. The results for clear sky comparisons suggest that a satellite observing system such as TOMS, which
provides global coverage daily, might partially serve as a fi rst-order check to quality assure UV observations being made by
networks worldwide. Future research should concentrate on determining the causes of the large differences seen between
the UVB-1 and TOMS and the range of uncertainties, using a larger array of stations.